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Gaining power – the dreaded ‘too few participants’

Gaining power – the dreaded ‘too few participants’

Statistical power is about whether an experiment or study is likely to reveal an effect if it is present. Without a sufficiently ‘powerful’ study, you risk being in the middle ground of ‘not proven’, not being able to make a strong statement either for or against whatever effect, system, or theory you are testing.

In HCI studies the greatest problem is often finding sufficient participants to do meaningful statistics. For professional practice we hear that ‘five users are enough’, but less often that this figure was based on particular historical contingencies and in the context of single iterations, not summative evaluations, which still need the equivalent of ‘power’ to be reliable.

However, power arises from a combination of the size of the effect you are trying to detect, the size of the study (number of trails/participants) and the size of the ‘noise’ (the random or uncontrolled factors).

Increasing number of participants is not the only way to increase power and we will discuss various ways in which careful design, selection of subjects and tasks can increase the power of your study albeit sometimes requiring care in interpreting results. For example, using a very narrow user group can reduce individual differences in knowledge and skill (reduce noise) and make it easier to see the effect of a novel interaction technique, but also reduces generalisation beyond that group. In another example, we will also see how careful choice of a task can even be used to deal with infrequent expert slips.